1. Technical Field
The present disclosure relates to a multilayer circuit board structure, in particular, to a multilayer circuit board structure adapted to suppress the undesired electromagnetic wave propagation within a specific frequency band.
2. Description of Related Art
Recently, consumer electronic device developed rapidly toward miniaturization, leading in increasing demands of faster clock rate, low voltage level, and high system integration. Consequently, power integrity circuit design becomes a major challenge in maintaining the stability of a system. Electromagnetic noise such as simultaneously switching noise (SSN) or ground bounced noise (GBN) will be induced as the transient switching current of the digital transistors flows through the parasitics of the power distribution network (PDN), as the result the output signal of the radio frequency (RF) circuit become severely distorted, and thereby effecting the stability of the operating system.
Recently, as illustrated in FIG. 1, a high impedance surface (HIS) electromagnetic bandgap (EBG) structure 10 has developed as a possible solution for mitigating SSN coupling noise on parallel power planes. The HIS EBG structure 10 is designed to mitigate the coupling electromagnetic noise within the desired bandgap above gigahertz range, so as to suppress the electromagnetic wave propagation of the undesired electromagnetic wave omni-directionally. Please refer to FIG. 1 in conjunction with FIG. 2, which provides a side view diagram of a HIS EBG structure. The HIS EBG structure 10 has a structure consisting of a plurality of mushroom shaped structure crystals 12. The plurality of mushroom shaped EBG structure crystals 12 are disposed periodically, and each of the mushroom shaped EBG structure crystals 12 has a via 140, an inner metallic conducting patch 120, a partition of the first outer metallic conducting plane 110, and a partition of the second outer metallic conducting plane 130. The inner metallic conducting patch 120 is connected to the partition of the second outer metallic conducting plane 130 through the via 140. The inner metallic conducting patch 120 and the via 140 are disposed between the first outer metallic conducting plane 110 and the second outer metallic conducting plane 130.
Further, a first and a second dielectric layers (not shown) are interposed between the first outer metallic conducting plane 110 and the inner metallic conducting patches 120, and between the inner metallic conducting patches 120 and the second outer metallic conducting plane 130. Consequently, the stack of the partition of the first outer metallic conducting plane 110, the inner metallic conducting patch 120, and the partition of the second outer metallic conducting plane 130 form a capacitive element. The capacitive element is connected in series with an inductive element formed by the via 140, and the HIS EBG structure 10 thereby functions as a band stop filter. The bandwidth parameters (such as an upper and a lower cutoff frequency) are therefore associated with the geometric parameters of the HIS EBG structure 10.
For instance, the side length of the partition of the first and the second outer metallic conducting planes 110, 130 is 4 mm (i.e. a=4 mm), the side length of the inner metallic conducting patch 120 is 3.8 mm (i.e. p=3.8 mm), and the radius of the via 140 is 37.5 μm. The separation between the partition of the first outer metallic conducting plane 110 and the inner metallic conducting patch 120 is 45 μm (i.e. h=45 μm), and the separation between the inner metallic conducting patch 120 and the partition of the second outer metallic conducting plane 130 is 90 μm. A corresponding dispersion diagram is depicted in FIG. 3, wherein a dispersion diagram generally describes the propagation of the electromagnetic wave over a set frequency range. The frequency bandgap in the presented example is from 3.05 GHz to 5.85 GHz, i.e., the energy gap between a first mode electromagnetic transmission curve C12 and a second mode electromagnetic transmission curve C14.
Despite that the HIS EBG structure 10 may effectively suppress the SSN noise within a desire band, the HIS EBG structure 10 as described does not yield broad stop band bandwidth. Further as shown in FIG. 1, the HIS EBG structure 10 requires large area occupation, and consequently might not be the most optimum solution to be adopted in the power integrity design for electromagnetic noise suppression.